Output short circuit has become an important item in the safety test of power supplies. Commercial power management integrated circuits (ICs) are all provided with built-in protection mechanism against output short circuit of power supplies. For example, as shown in FIG. 1, in a typical flyback power supply 10, the input alternating-current (AC) voltage Vac is first rectified and filtered by a diode bridge 12 and a capacitor 14 to a direct-current (DC) input voltage Vin to apply to a primary coil Lp of a transformer 18, and a power switch 20 is serially connected to the primary coil Lp and switched by a control signal Gate_BM provided by a power management IC 16 for the transformer 18 to deliver power from the primary coil Lp to a secondary coil Ls to thereby generate an output voltage Vout at a power output. A current sense resistor 22 is connected in series to the power switch 20 to sense the current Ip of the primary coil Lp, and the power management IC 16 determines the duty of the power switch 20 according to the current sense signal Vcs from the current sense resistor 22 and a feedback signal Vcomp derived from the power output Vout. Without any additional pins, the power management IC 16 usually carries out the detection for output short circuit of the flyback power supply 10 by using the feedback pin COMP or the power pin VDD, for triggering the built-in protection mechanism thereof. However, the information provided by the feedback pin COMP contains the message of the feedback open circuit, and the power pin VDD is connected to a capacitor, so that the output information cannot be immediately reflected. Conventionally, regardless of using the feedback pin COMP or the power pin VDD in the detection for output short circuit, it always takes tens to hundreds of milliseconds to determine if an output short circuit occurs. When the flyback power supply 10 suffers output short circuit, it will blowout a large current, which will induce considerable power consumption if the detection for output short circuit needs a long time, and thereby cause the flyback power supply 10 overheated.
Therefore, it is desired a method of quick and immediate detection for output short circuit of a flyback power supply.